Low pressure gas discharge switch

ABSTRACT

For a low-pressure gas discharge switch, at least two main electrodes are arranged at least a distance d from each other, the electrodes in an arcing chamber forming a cathode and an anode of a discharge path for the low-pressure gas discharge. The gas discharge is triggered by increasing the electron density in a cathode cavity, at least the cathode in its disk-shaped area having at least one aperture, the cathode and anode apertures preferably being opposite and aligned with each other, for triggering the discharge. An arrangement generating a magnetic field superimposed on the discharge between the main electrodes ( 1, 1   a   , 2, 2   a ) are assigned to the main electrodes ( 1, 1   a   , 2, 2   a ), with which either a predominantly parallel magnetic field is generated or a predominantly perpendicular one, with regard to the direction of current in the discharge. The magnetic field generator may include slot arrangements ( 11, 11′, 21, 21 ′) in hollow cylinders ( 1   b   , 2   b ), which are part of the anode ( 1 ) and cathode ( 2 ) configured as hollow electrodes, or may be realized in the associated current supply lines.

BACKGROUND INFORMATION

The present invention relates to a low pressure gas discharge switch, inwhich, for a low-pressure gas discharge, main electrodes are arranged atleast at a distance d from each other. The electrodes in an arcingchamber form a cathode and an anode of a discharge path for thelow-pressure gas discharge that is triggered by increasing the electrondensity in a cathode cavity. At least the cathode in its disk-shapedarea has at least one aperture for triggering the discharge. The cathodeand anode apertures being opposite, and aligned with, each other. Anarrangement for generating a magnetic field is assigned to the mainelectrodes.

Low-pressure gas discharge switches for switching of high pulse-shapedcurrents and power outputs are essentially composed of at least two mainelectrodes, of which at least the cathode has one or a plurality ofapertures which are designated as trigger apertures. Via this (these)aperture(s), the area between the main electrodes is connected to thearea behind the cathode. In this cathode rear space, a trigger device isgenerally arranged, with whose assistance electrons are released whichinitiate, i.e., trigger, the necessary main discharge in the areabetween the anode and cathode, to close the switch.

In switches having thermionic electron generation, i.e., a thyratron,there is, in the cathode rear space, an electrically heated electrodewhich not only makes the necessary electrons available for triggeringthe main discharge, but also supplies the greater part of the overallcurrent during the main discharge and thus acts as a thermionic cathode.After each use of the thyratron, however, a significant part of thecurrent continues to flow via the cold cathode and, as a result ofvaporization and atomization of the electrode material, leads to erosionof the material.

In low-pressure gas discharge switches, such as those described in WO89/00354 A1 or German Patent No. 28 04 393 A1, the entire current of thehigh-current main discharge flows via the cold cathode and leads thereto increased erosion, which, in widening the trigger apertures, leads tothe destruction of the cathode and thus to the end of the service lifeof the switching tube. The erosion, within certain limits, isproportional to the entire charge quantity transported by the switch,the quantity thus decisively influencing the service life of the switch.In response to high current densities, i.e., given a small cross-sectionof the discharge, the erosion rate increases disproportionately; inaddition, if the discharge has a small discharge cross-section, a highlocal volume erosion rate has significantly greater effects than if thedischarge has a large discharge cross-section.

The main problem for achieving a long service life of such switchingsystems is therefore to make the discharge cross-section as large aspossible and to provide for a homogeneous current distribution over theentire discharge cross-section. In this way, the erosion is reducedlocally and, overall, is distributed equally over a larger surface, sothat the result is a uniform wearing away of the electrode instead oflocally pronounced erosion. Furthermore, by increasing the dischargecross-section it is achieved that the greater part of the vaporized oratomized electrode material is deposited again on the electrodeopposite, so that by increasing the discharge cross-section, adisproportionate reduction of the macroscopically detectable erosion canbe achieved.

Low-pressure gas discharge switches are known in various specificembodiments. Specific embodiments having only one, particularly round,aperture in the cathode are also called pseudo-spark switches, and aredescribed in, e.g., WO 89/00354 A1 and German Patent No. 28 04 393 A1.Especially in the switches shown there, there is an aperture in theanode that is identical to, and aligned with, the aperture in thecathode, for maintaining a symmetrical arrangement independent ofpolarity.

The parallel connection of such individual discharge channels forreducing the load of the individual switching channel is conventional,and it is specifically described in the specialized literature foraccommodating the individual channels in a common vacuum housing. Theaccommodation of individual discharge channels in separate housings isalso conventional. Slot-shaped apertures for enlarging the electrodesurface are described in German Patent No. 42 40 198 C1. Finally, it isconventional to use a plurality of slot-shaped apertures in the cathodeof thyratrons which are operated predominantly using a cold cathode inthe so-called “grounded-grid” mode.

A further method for increasing the discharge cross-section is describedin detail in U.S. Pat. No. 5,146,141. There, the enlargement of thedischarge cross-section is achieved because in the cathode, instead ofan aperture, a recess is provided, over whose surface the dischargespreads out, given suitable geometry. The triggering of the discharge isachieved by holes in the edge region of the recess, which connect theactual discharge chamber, between the anode and the cathode, to thecathode rear space and a trigger device accommodated there.

Common to all hitherto known embodiments is that wear and tear takesplace through the erosion of the cathode, the erosion, at a time pointthat cannot be predicted in advance, becoming spatially inhomogeneous,i.e., becoming locally intensified.

This is particularly caused by the self-generated magnetic field of thedischarge, as a result of which the discharge tends to be constricted(so-called pinch effect). Since the discharge is triggered due to thelow working pressure, preferably assuming the largest aperture,therefore, given the existing asymmetry, a certain point, already in thetriggering of the discharge, is favored at which the discharge thenburns in a concentrated fashion and at which therefore the erosionbecomes more intense. In this way, this undesirable effect intensifiesand the service life of the switching element is prematurely limited bylocal erosion.

A gas discharge switch is also described in Japanese Patent No.5,159,851. In this gas discharge switch, a means for generating amagnetic filed in the switch is formed using slot arrangements. Theslots in the arrangement run in the same direction and are in the wallsof hollow electrodes. This means superimposes a parallel, i.e., axial(with respect to the direction of current in the discharge), magneticfield.

An objective of the present invention is to provide a low-pressure gasdischarge switch having a cold cathode such that the erosion,particularly of the cathode, is reduced.

In an first example embodiment of the present invention, main electrodesare provided having disk-shaped bottoms. These main electrodes may beprovided with radial slots for avoiding eddy current effects.Additionally, a magnetic field generator is provided which produces asubstantially parallel magnetic field, i.e., an axial field, withrespect to the direction of current in the discharge. An auxiliaryelectrode may also be provided for electrically triggering a switchingprocess.

In another example embodiment, the main electrodes are provided withdisk-shaped bottoms. These main electrodes are provided with slots thatrun substantially tangentially or in a spiral shape. The magnetic fieldgenerator produces a substantially perpendicular magnetic filed, i.e., aradial field, with respect to the direction of current in the discharge.An auxiliary electrode ma also be provided for electrically triggering aswitching process.

The means for generating the magnetic fields are preferably realizedthrough slot arrangements in the cylinders completing the anode, on theone hand, and the cathode, on the other hand. However, it is alsopossible to arrange the slot arrangements in the power supply conductorsto the cathode and/or the anode.

The predominantly axial or radial magnetic fields, with respect to thecircular symmetry of the low-pressure gas discharge switch, can beinfluenced by a corresponding tilt of the slots in the different partialunits or the arrangement of the permanent magnets or arrangement ofindividual coils of various types.

Arcs that are superimposed on magnetic fields and the associated meansfor generating such magnetic fields are already known in principle fromthe technology of vacuum switches. Especially in connection with gasdischarge switches, such magnetic fields surprisingly generateunexpected advantages, since the damaging erosion of the electrodes isreduced particularly for the continuous operation of a gas dischargeswitch.

The latter is possible as a result of the knowledge that in thelow-pressure range switching plasmas can be expanded, using magneticfields, two or three orders of magnitude more quickly than with theconventional arrangements of vacuum switches. Only in this way is theuse of magnetic fields superimposed on the switching plasma sensible inthe case of short pulses, since using the previously known expansionspeeds of switching plasmas in a vacuum, i.e., in vacuum arcs, nosignificant improvement of the switching performance and of the erosionof the electrodes can occur for short, high-current discharges.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a gas discharge switch in cross-section having a slotarrangement in a hollow cylinder supporting the cathode and the anode,the slot arrangements in both cylinders running in the same direction,according to an example embodiment of the present invention.

FIG. 2 shows the switch according to FIG. 1 in switching operationhaving a stationary arc that is diffusely formed.

FIG. 3 shows a low-pressure gas discharge switch in cross-sectioncorresponding to FIG. 1, in this case the slot arrangements in thehollow cylinders running in opposite directions.

FIG. 4 shows the switch according to FIG. 3 in switching operationhaving a concentrated arc rotating in a circle.

FIG. 5 shows a hollow electrode, cutaway in the front in a perspectiverepresentation, for clarifying the slot geometry.

DETAILED DESCRIPTION

In each case, in the individual Figures, an identically configuredlow-pressure gas discharge switch, in principle furnished with hollowelectrodes, is depicted as it is known in the Prior Art. Specifically,reference in this regard is made to the documents treated in theintroduction with regard to the Prior Art.

To achieve large discharge cross-sections in low-pressure gas dischargeswitches, experience teaches that an axial magnetic field superimposedon a discharge exercises a stabilizing effect on the discharge itselfand in certain cases prevents, or at least reduces, a constricting ofthe discharge to small cross-sections. In vacuum switches having movableelectrodes, it is known that an axial magnetic field superimposed on thearc exercises a stabilizing effect of this type, as a result of whichthe arc voltage of the arc discharge is reduced, and the arc can be keptin a diffuse condition over a larger cross-section. This magnetic field,inter alia, is generated because one or both contact carriers is/areconfigured as a coil. The arc in vacuum switches is produced bymechanically separating the current-conducting contact pieces touchingeach other, the expansion of the arc over a larger cross-section takingplace via the expansion of the metal vapor arising in the discharge andvia the ignition of new cathode base points in areas of sufficientlyhigh metal vapor density.

In contrast, in low-pressure gas discharge switches the ignition of thedischarge between the stationary electrodes is initiated by injection offree charge carriers, i.e., electrons. The formation of plasmaconsequently takes place largely in the working gas. An expansion of thedischarge cross-section in the working gas is therefore not dependent onthe expansion of the metal vapor and can therefore proceed significantlymore quickly. Due to the presence of a preselected low-pressure gasfilling, the electrode erosion is significantly reduced, since the gasfilling replaces a significant part of the vapor density necessary forcurrent transport.

Whereas in vacuum switches having movable contacts a pressure upperlimit of approximately 10⁻³ Pa is indicated in the literature as thecorrect mode of functioning for switching off high currents, the optimalpressure for the function of switching on high currents required by thepresent invention is typically in the range of between approximately 1Pa and 200 Pa, given stationary electrodes at a distance typically ofsome mm. The knowledge that axial magnetic fields, in this pressurerange and in current flow durations of only a few microseconds, have astabilizing and homogenizing effect on the discharge plasma, issurprising in this connection and can be exploited for particularlyadvantageous solutions.

In FIG. 1 and FIG. 2, an example embodiment of a low-pressure gasdischarge switch that can be triggered from the outside, i.e.,triggerable, is depicted in detail, in which a stabilization of the arcis achieved through an axial magnetic field generated in the powersupply conductor area. The switch is composed of two stationary,rotationally symmetric, and cup-shaped electrodes 1 and 2, each composedof a “cup” bottom 1 a and 2 a having distance d between them, and ahollow cylindrical “cup” wall 1 b and 2 b. In this context, electrode 1realizes the anode and electrode 2 realizes the cathode for thedischarge.

In FIG. 1, both electrode cylinders 1 b and 2 b have a slot arrangement,composed of at least two transverse slots 11 and 21, respectively. Slots11 and 21, in this context, are distributed equally over the peripheryand constitute, for each cylinder wall 1 b and 2 b, at least one entirewinding.

The number and angle of slots 11 and 21 determine the strength of theproportion of axial magnetic field created in the axle area. To reducethe eddy current moving in the opposite direction inevitably produced inelectrode bottoms 1 a and 2 a, it is expedient to provide these areaswith slots that have a radial component. Thus a reduction of the axialmagnetic field is avoided.

At least cathode 2 has an aperture 22 in the axis area, the apertureconnecting the side of the cathode turned toward the anode to theso-called cathode rear chamber, which forms a hollow cathode 23. Theaperture is composed, for example, of a circular bore having a diameterof approximately 2 to 10 mm; but annular apertures are also possible.

Electrodes 1 and 2 are located in a gas-tight, closed housing 3 and aresupported by an annular tube segment 31 made of insulating material at apreselected distance of typically 2 to 8 mm. The entire area withinhousing 3 is filled with an ionizable gas filling in the pressure rangebetween 1 and 200 Pa. Suitable for the gas are hydrogen or deuterium ora mixture of them, which can be stored, in accordance with the PriorArt, in metal hydride storage chambers and released selectively bywarming up the storage chamber.

In the operating condition of the switch, the gas pressure is adjustedso that the gas path in all the areas between anode 1 and cathode 2resists the applied voltage, i.e., is electrically insulating (“open”),and no independent discharge can occur. The switch is closedelectrically by the fact that in gap 39 between anode bottom 1 a andcathode bottom 2 a a discharge plasma is produced which connects, in anelectrically conductive manner, anode 1 and cathode 2 as mainelectrodes.

Using a trigger electrode 30, the discharge is triggered by producing asufficient number of free charge-carriers in hollow cathode 23.Typically, approximately 10⁸ through 10¹¹ free electrons are requiredwithin a time period of from 10 to 100 ns. For generating the triggerelectrons, a series of methods is known: for example, pulsed gasdischarges or a pulsed extraction from stationary gas discharges, pulsedcorona discharges, pulsed creeping discharges on insulator surfaces,thermionic cathodes, external photoelectric effect, ferroelectricelectron sources, among others, can be used. The trigger electrons leadto creating a transient hollow cathode discharge in hollow electrodearea 23, i.e., a gas discharge, whose discharge plasma expands from area23 into the area between the anode and cathode and connects the twoelectrodes 1 and 2 in an electrically conductive manner. An arc-like,diffuse discharge, in this context, is promoted by the symmetry of thedischarge.

The supply of the discharge current to the area of the discharge in thecenter of electrodes 1 and 2 occurs due to the cup-shaped structure ofelectrodes 1 and 2 always via the bars of the coil winding formed fromslots 21 and 11. In this way, in the central area of electrodes 1 and 2,a predominantly axial magnetic field is produced. This magnetic fieldprevents the discharge plasma, in particular at high currentintensities, from contracting to a discharge channel of a small diameterdue to the pinch effect, in that the interior plasma pressure iscorrespondingly increased by “freezing” the axial field. In this way, adiffuse discharge having low local erosion rates is achieved even athigh current intensities of over 40 kA, whereas otherwise it is knownthat the discharge at high current intensities has a tendency to build adense, contracted metal vapor arc having arc erosion rates that arehigher by orders of magnitude.

For raising the magnetic field intensity of the axial field, it isadvantageous to provide the anode with slots 11 which have the same slotdirection as the slots of cathode 21. In this way, the axial fieldcomponents of anode 1 and cathode 2, in the contact gap and in dischargearea 39, are superimposed in the same direction, which raises the totalaxial field intensity. In this context, cathode aperture 22 and acomparable anode aperture 12 have a diameter that is typically roughlyone magnitude smaller than the external diameter of cup-shapedelectrodes 1 and 2.

FIG. 2 makes clear how arc diffuse stationary arc [Diffuser StationaererLichtobgen] (DSL), stabilized by the axial magnetic field, is formed ina diffuse, homogeneous manner around electrode apertures 12 and 22 andis stationary in this condition. For avoiding eddy current effects, atleast one of the disk-shaped electrode bottoms 1 a or 2 a can have slotspredominantly in the radial direction, as can be seen by way of exampleas radial slots 32 in FIG. 5.

In the specific embodiment according to FIGS. 3 and 4, in place of theaxial magnetic field a radial magnetic field is used in contact gap 39between anode 1 and cathode 2, to place into an azimuthal rotatingmotion the arc commutated at the area of the edge of aperture 22 and 12.In this way, the local effect of the arc especially in current pulses oflong duration are spread evenly over a large area, and in this way adisproportionately intense, locally damaging effect on the electrodes isavoided.

In order to achieve the latter, in accordance with FIGS. 3 and 4, theelectrode cylinders 1 b and 2 b are slotted in opposite directions.Alternatively, to generate a radial magnetic field, disk-shaped bottoms1 a and 2 b of anode and cathode can have slots, not depicted in detail,which run predominantly tangentially or in a spiral shape.

FIG. 4 especially clarifies how a concentrated arc circular runningconcentrated arc [Kreisfoermig umlaufender Konzentrierter Lichtbogen](KKL), in a specific embodiment according to the present invention,moves in a circular motion around the electrode bores due to the radialmagnetic field, as a result of which local damage to the electrode disksis avoided. In this way, in particular at currents of high amplitudes ofseveral tens of kA to over 100 kA and at long current flow durations,equalized utilization of the surface and a long service life of theelectrodes are achieved.

Alternatively, for generating a radial magnetic field, disk-shapedbottoms 1 a and 2 b of anode and cathode can have slots which runpredominantly tangentially or in a spiral shape.

In FIG. 5, a single hollow electrode 1 and 2, for use as cathode andanode, respectively, is clarified in a front cutaway view in a gasdischarge switch in accordance with FIG. 1 and FIG. 3. Apart from radialslots 32, the geometry of the slot arrangement for producing themagnetic field is particularly obvious, the angle of slots 11, 11′ and21, 21′ with respect to the vertical being represented by α and theazimuth angle of an individual slot with respect to the periphery beingrepresented by β. Length l of a single slot is dependent on the angleposition and the height h of the coil. The intensity of the magneticfield is determined, assuming n slots, by overall length L, wherein L isthe sum of the single slots of length l. For assuring sufficientmagnetic field intensity, the following should hold:${L = {{\sum\limits_{n}\quad l} > {2\pi \quad r}}};{and}$n ⋅ β > 360^(∘).

Therefore, a sufficient axial or radial magnetic field can be generatedby at least one complete coil winding.

Research has shown that the axial magnetic field, for use in the gasdischarge switch described, should be at least 1 mT per kA of thecurrent to be switched, and the radial magnetic field for use in the gasdischarge switch described should be 2 mT per kA of the current to beswitched, but at least 30 mT.

The material, at least for bottoms 1 a and 1 b of hollow electrodes 1and 2 in the gas discharge switches according to FIGS. 1 and 3, iscomposed advantageously of a copper-chrome (CuCr) alloy. The materialCuCr40 has been shown to be particularly suitable for minimizing theerosion at the discharge aperture.

What is claimed is:
 1. A low-pressure discharge switch, comprising: mainelectrodes arranged at a distance from each other, the main electrodeshaving disk-shaped bottoms, the disk-shaped bottoms having substantiallyradial slots for avoiding eddy current effects; an arcing chamber, themain electrodes being positioned within the arcing chamber and forming acathode and an anode of a discharge path for a low-pressure gasdischarge, the cathode having an aperture, the discharge being triggeredby increasing an electron density in the at least one aperture of thecathode, the anode including an aperture, the aperture of the anode andthe aperture of the cathode being opposite and aligned with each other;a magnetic field generator assigned to the main electrodes, the magneticfield generator generating a magnetic field superimposed on thedischarge, the magnetic field being substantially parallel with respectto a direction of current in the discharge; and an auxiliary electrodeassociated with the cathode, the auxiliary electrode electricallytriggering a switching process.
 2. The low-pressure gas discharge switchaccording to claim 1, wherein the magnetic field generator is formed bya slot arrangement in cylinders, the cylinders forming a portion of theanode and the cathode.
 3. The low-pressure gas discharge switchaccording to claim 2, wherein slots in the slot arrangement in thecathode cylinder and slots in the slot arrangement in the anode cylinderare tilted in the same direction.
 4. The low-pressure gas dischargeswitch according to claim 2, wherein the magnetic field depends on anumber and an angle of slots in the slot arrangement, the magnetic fieldfurther depending on an overall length, the overall length beingdetermined using the following formulae:${L = {{\sum\limits_{n}\quad l} > {2\pi \quad r}}};{and}$n ⋅ β > 360^(∘).

where L is the overall length, l is the length of one slot, r is theradius of a cylinder, n is the number of slots and β is the peripheralangle of a single coil segment defined by the slots.
 5. The low-pressuregas discharge switch according to claim 1, wherein the magnetic fieldgenerator is formed by a slot arrangement of current supply lines to atleast one of the cathode and the anode.
 6. The low-pressure gasdischarge switch according to claim 5, wherein the magnetic fielddepends on a number and an angle of slots in the slot arrangement, themagnetic field further depending on an overall length, the overalllength being determined using the following formulae:${L = {{\sum\limits_{n}\quad l} > {2\pi \quad r}}};{and}$n ⋅ β > 360^(∘).

where L is the overall length, l is the length of one slot, r is theradius of a cylinder, n is the number of slots and β is the peripheralangle of a single coil segment defined by the slots.
 7. The low-pressuregas discharge switch according to claim 1, wherein an intensity of themagnetic field is at least 1 mT per kA in current to be switched.
 8. Thelow-pressure gas discharge switch according to claim 1, wherein thebottoms of the main electrodes include a copper-chromium material. 9.The low-pressure gas discharge switch according to claim 1, wherein aproduct of a pressure of gas in the switch and the distance between themain electrodes in the switch is less than 200 Pa·mm and more than 1Pa·mm.
 10. A low-pressure discharge switch, comprising: main electrodesarranged at a distance from each other, the main electrodes havingdisk-shaped bottoms, the disk-shaped bottoms having one of i)substantially tangentially running slots, and ii) slots which run in aspiral shape; an arcing chamber, the main electrodes being positionedwithin the arcing chamber and forming a cathode and an anode of adischarge path for a low-pressure gas discharge, the cathode having anaperture, the gas discharge being triggered by increasing an electrondensity in the at least one aperture of the cathode, the anode includingan aperture, the aperture of the anode and the aperture of the cathodebeing opposite and aligned with each other; a magnetic field generatorassigned to the main electrodes, the magnetic field generator generatinga magnetic field superimposed on the discharge, the magnetic field beingsubstantially perpendicular with respect to a direction of current inthe discharge; and an auxiliary electrode associated with the cathode,the auxiliary electrode electrically triggering a switching process. 11.The low-pressure gas discharge switch according to claim 10, wherein themagnetic field generator is formed by a slot arrangement in cylinders,the cylinders forming a portion of the anode and the cathode.
 12. Thelow-pressure gas discharge switch according to claim 11, wherein slotsin the slot arrangement in the cathode cylinder and slots in the slotarrangement in the anode cylinder are tilted in the opposite directions.13. The low-pressure gas discharge switch according to claim 11, whereinthe magnetic field depends on a number and an angle of slots in the slotarrangement, the magnetic field further depending on an overall length,the overall length being determined using the following formulae:${L = {{\sum\limits_{n}\quad l} > {2\pi \quad r}}};{and}$n ⋅ β > 360^(∘).

where L is the overall length, l is the length of one slot, r is theradius of a cylinder, n is the number of slots and β is the peripheralangle of a single coil segment defined by the slots.
 14. Thelow-pressure gas discharge switch according to claim 10, wherein themagnetic field generator is formed by a slot arrangement of currentsupply lines to at least one of the cathode and the anode.
 15. Thelow-pressure gas discharge switch according to claim 14, wherein themagnetic field depends on a number and an angle of slots in the slotarrangement, the magnetic field further depending on an overall length,the overall length being determined using the following formulae:${L = {{\sum\limits_{n}\quad l} > {2\pi \quad r}}};{and}$n ⋅ β > 360^(∘).

where L is the overall length, l is the length of one slot, r is theradius of a cylinder, n is the number of slots and β is the peripheralangle of a single coil segment defined by the slots.
 16. Thelow-pressure gas discharge switch according to claim 10, wherein anintensity of the magnetic field is at least 2 mT per kA in a current tobe switched, and at least 30 mT.
 17. The low-pressure gas dischargeswitch according to claim 10, wherein the bottoms of the main electrodesinclude a copper-chromium material.
 18. The low-pressure gas dischargeswitch according to claim 10, wherein a product of a pressure of gas inthe switch and the distance between the main electrodes in the switch isless than 200 Pa·mm and more than 1 Pa·mm.